A well-known use of phosphors is in the production of X-ray images. In a conventional radiographic system an X-ray radiograph is obtained by X-rays transmitted image-wise through an object and converted into light of corresponding intensity in a so-called intensifying screen (X-ray conversion screen) wherein phosphor particles absorb the transmitted X-rays and convert them into visible light and/or ultraviolet radiation to which a photographic film is more sensitive than to the direct impact of X-rays.
In said conventional radiography (“X-ray photography”), a film plate is made by forming one or more silver halide emulsion layers on a flexible film base which is supported within a light-tight cassette. The interior of the cassette is coated with one or more X-ray sensitive luminescent layers. The cassette containing an unexposed X-ray film plate is loaded into an X-ray machine, and after exposure the cassette and exposed X-ray film plate are removed for development and fixing of the latent image produced. This is usually done automatically by feeding the cassette into a light-tight apparatus in which the cassette is opened, and the exposed film plate is extracted and passed through a series of troughs containing the various chemical processing solutions as required. The processed plate may also be dried in the apparatus. Meanwhile, a new, unexposed film plate has been loaded into the cassette which is then re-closed, and the reloaded cassette and developed film plate are delivered to respective exit slots of the processing apparatus.
According to another method of recording and reproducing an X-ray pattern disclosed e.g., in U.S. Pat. No. 3,859,527, a special type of phosphor is used, known as a photostimulable phosphor, which being incorporated in a panel or screen, is exposed to incident pattern-wise modulated X-ray beam and, as a result thereof, temporarily stores energy contained in the X-ray radiation pattern. At some interval after the exposure, a beam of visible or infra-red light scans the panel or screen in order to stimulate the release of stored energy as light that is detected and converted to sequential electrical signals which can be processed to produce a visible image. For this purpose, the phosphor should store as much as possible of the incident X-ray energy and emit as little as possible of the stored energy until stimulated by the scanning beam. This is called “photostimulated light—PSL—radiography”, “digital radiography” or “Computed Radiography” (CR).
Current practice in “photostimulated ligth radiography” has been to pass the exposed PSL plate in its cassette to an automatic processing machine in which the PSL plate is removed from the cassette, scanned, exposed overall to light in order to return the PSL material to its ground state and then reloaded into the same cassette for reuse. For scanning, the exposed PSL plate is transported past a laser, which scans line-wise across the plate in front of a light-guide comprising a bundle of optical fibres whose input ends are arranged in a line across the path of the plate close to the laser scanning line for the reception of light emitted, typically at wavelengths close to 400 nm, when the PSL material is stimulated by the laser. The light-guide is arranged to pass the emitted light to a photo-multiplier tube or other receptor. The result is a storable electronic raster image. The electronic image may be subjected to any desired computer image-enhancement techniques and it may be displayed on a video display unit, fed to a laser printer for the production of a plain paper copy, or used to control a laser arranged to expose correspondingly a photographic film plate to produce an X-ray plate of conventional appearance.
In U.S. Pat. No. 5,340,995 there has been provided a scanning apparatus for scanning a cassette of the type used in photo-stimulable luminescence (“PSL”) radiography, which cassette comprises a flat substantially rigid base plate which is releasably securable to the base plate so as light-tightly to cover a layer of PSL material applied to a face of the base plate, characterized in that such apparatus comprises a receiving station for the receipt of a cassette into the apparatus, transport means for conveying the cassette to a separating station which includes means for separating the base plate and its cap from each other, means for transporting the base plate along a path leading through a scanning station where the plate may be scanned and, via an erasing station, to an assembly station where the plate and its cap are re-assembled, the apparatus being arranged in such a way that the cap and the base plate remain in substantially parallel relationship during their separation. The arrangement thus avoids flexure of the layer of PSL material on the plate and offers a compact construction.
In U.S. Pat. No. 6,373,074 an advanced device has been described for the line by line read out of information stored in a phosphor carrier with a radiation source that can generate several individual beams, in order to emit a primary radiation providing ability to stimulate the phosphor carrier such that it emits a secondary radiation that contains at least a partial reproduction of the stored information. A receiving device for point by point reception of the secondary radiation emitted by the phosphor carrier includes a multitude of point elements, wherein the secondary radiation that is emitted by the phosphor carrier can be received at the same time by a plurality of these point elements, wherein the radiation source includes an optical device for expanding the several individual beams in the direction of a line on the phosphor carrier. Furtheron the device comprises reproduction means, located between the phosphor carrier and the receiving device, for imaging the secondary radiation emitted by the individual points of the phosphor carrier in a ratio of 1:1 on the individual point elements.
An X-ray cassette has moreover been claimed in the same U.S. Pat. No. 6,373,074 for writing to a phosphor carrier contained in the cassette, the improvement wherein the cassette includes a radiation source for emitting a primary radiation that can be used to stimulate the phosphor carrier such that it emits a secondary radiation for line-by-line read out of information stored in the phosphor carrier, wherein said secondary radiation contains at least a partial image of the stored information, and wherein the cassette includes a receiving device for point-by-point reception of the secondary radiation emitted by the phosphor carrier, wherein the receiving device contains a multitude of point elements and where the secondary radiation emitted by the phosphor carrier can be received by several of these point elements at the same time.
In EP-A 1 130 417 and US-Application 2001/0017356 a system for reading a radiation image has been described, said system comprising an array of imaging elements arranged to detect said radiation image and to convert it into a charge representation of said image, as well as charge integrating means coupled to said array of imaging elements for integrating an amount of charge detected by an element of said array characterized by means for determining or setting a charge amount which is expected to be detected, means for adjusting the charge storage capacity of said charge integrating means in accordance with the expected charge amount.
It is clear that the image quality produced by any radiographic system using a phosphor screen, thus also by a computer radiography (CR) system, depends largely on the construction of the phosphor screen. Generally, the thinner a phosphor screen at a given amount of absorption of X-rays, the better the image quality will be. This means that the lower the ratio of binder to phosphor of a phosphor screen, the better the image quality, attainable with that screen, will be. Optimum sharpness can thus be obtained when screens without any binder are used. Such screens can be produced, e.g., by physical vapor deposition, which may be thermal vapor deposition, sputtering, electron beam deposition or other of phosphor material on a substrate.
Good image quality also implies that the sensitivity and the sharpness of the system is constant over the image area. I.e. when a CR screen is scanned in a CR scanner after a flat-field exposure, the signal should be as homogeneous as possible. In order to achieve this goal a screen with a homogeneous sensitivity should be used. It is necessary as well, however, to have a scanning system that is as constant in quality as possible. An important parameter that influences the quality consistency of the system is the distance between the CR screen phosphor layer and the light detector in the CR scanner. The light collection efficiency of the light detector critically depends on the distance between the phosphor layer and the light detector. This is the case when the light detector in the scanner consists of a photomultiplier tube (PMT) and a light guide to guide the emission light to the PMT as is the case in a flying-spot scanner. This is even more so when the light detector in the scanner consists of a CCD array and a lens system (SELFOC or microlens) in order to project the emission light of the phosphor screen onto the CCD elements as in a scanner scanning line-wise or two-dimensional-area wise. In general, the larger the distance between the phosphor layer and the light detector, the lower the sensitivity of the CR system.
Since the optical system in the scanner in general has a limited sharpness depth also sharpness will be affected by the distance between the CR screen and the light detector in the CR scanner. Having a variable distance between screen and light detector causes the sharpness of the image to vary over the image area, which is evidently not allowed.
The only practical way of making the distance between the phosphor layer and the light detector as constant as possible is by having a light detector that is as flat as possible and a screen that is as flat as possible and by moving the flat surface of the detector over the flat surface of the screen at a constant distance. A good way of having a flat phosphor surface for scanning is by having a plate that is constant in thickness and by pushing or pulling the plate onto a very flat surface. An excellent way to achieve this is by pulling the plate onto a flat-bed in the scanner by vacuum suction.
If the edges or corners of the screen are upstanding when the screen is placed onto the vacuum table, air leaks always exist at the upstanding side and the space between the screen back and the vacuum table cannot be evacuated, leading to no vacuum. Likewise, if the curvature of the screen is too large, a spacing will exist between the screen back and the vacuum table leading to air leaking and loss of vacuum.
It is clear from the background as set forth above that reading out a stimulable phosphor panel having needle-shaped phosphors requires stringent demands from the point of view of flatness of the flat panel as otherwise light escapes, resulting in loss of speed (sensitivity) and image definition (sharpness). The technology as set forth even tolerates a curvature of not more than 100 μm. A “flat” storage phosphor panel as such does not provide such a reduced curvature, so that a solution therefor is highly requested.